1. Field of the Invention
The present invention relates to an image stabilizer for correcting an image blur due to hand-held shooting.
2. Description Related to the Prior Art
Many optical instruments including digital still cameras have an image stabilizer or a shake correction device for correcting an image blur caused by vibration or a camera shake due to hand-held shooting. The image stabilizer is constituted of a shake detector for detecting the camera shake, a holder for movably holding an optical element such as a taking lens or an image sensor, a computing unit, an actuator for shifting the holder in a plane where the optical element is orthogonal to a photography optical axis, and a position detector for detecting the position of the optical element. The computing unit calculates which direction and how much the optical element should be shifted, based on a detection result of the shake detector such as an angular velocity sensor. The actuator performs feedback control of the shift of the optical element during the camera shake, by using a calculated shift amount as a target value and a present position detected by the position detector as a measurement value.
In the conventional image stabilizers, the holder holds the optical element by guide shafts and bearings in a slidable manner in a direction orthogonal to the photography optical axis. However, wobble and large friction occurring between the guide shaft and the bearing degrades the followability of the holder to the actuator. Accordingly, there is disclosed the type of image stabilizer in which the optical element is held by elastic members disposed along the photography optical axis to reduce the friction during the shift of the optical element (for example, Japanese Patent Laid-Open Publication No. 02-066536, U.S. Pat. No. 7,161,621, and United States Patent Application Publication Nos. 2007/0172220, 2005/0110873 and 2007/0047935). This type of image stabilizer, however, is large in size in the direction of the photography optical axis.
To downsize the image stabilizer in the direction of photography optical axis with reduction in the friction, there are disclosed image stabilizers in which the optical element is held by a single or plurality of leaf springs that is bendable in a plane orthogonal to the photography optical axis. The image stabilizer according to Japanese Patent Laid-Open Publication No. 06-046314 has two pairs of leaf springs for holding an image sensor. The leaf springs of each pair are disposed in parallel with each other. The two pairs of leaf springs are elastically bendable in directions orthogonal to each other. Both ends of each individual leaf spring are attached to fixed or movable pieces.
According to the image stabilizer of U.S. Pat. No. 7,327,952, two pairs of leaf springs are integrally made of plastic or integrated into one body by insert molding. There is also described an embodiment in which the two pairs of leaf springs are integrated by U-shaped metal leaf springs.
According to the image stabilizer of United States Patent Application Publication No. 2007/0297005, a leaf spring strip is bent into a rectangular leaf spring frame. The leaf spring frame is fixed to a lens barrel at two horizontal sides, and holds a circuit board having an image sensor with two vertical sides. On the upper side of the leaf spring frame, a magnet is attached. The magnet and a coil attached to the circuit board constitute a voice coil motor. The voice coil motor deforms the leaf spring frame, and shifts the circuit board in the plane orthogonal to the photography optical axis.
In any of the above image stabilizers using the leaf springs, the leaf springs of each pair have to be disposed in parallel with each other with high precision. If the leaf springs are not in parallel, one leaf spring inhibits elastic bending of the other leaf spring, and interferes with the smooth shift of the optical element. The image stabilizer according to the Japanese Patent Laid-Open Publication No. 06-046314, however, has difficulty in precisely placing the pairs of leaf springs in parallel due to the adverse effects of dimensional accuracy of the fixed and movable pieces and attachment accuracy of the leaf springs.
The plastic leaf springs disclosed in the U.S. Pat. No. 7,327,952 are manufactured by injection molding. Thus, the leaf springs have a limitation in reducing their thicknesses, and cannot easily have appropriate elasticity. In the insert molding, the pair of leaf springs is hard to precisely set in a mold in parallel with each other. Even with the use of the U-shaped leaf spring, the pair of leaf springs is still hard to precisely dispose in parallel with each other, because two parallel sides of the U-shaped leaf spring tend to be open.
In the image stabilizer according to the United States Patent Application Publication No. 2007/0297005, the voice coil motor has to precisely generate a driving force with respect to the center of gravity of the rectangular leaf spring frame, in order to shift the circuit board in the plane orthogonal to the photography optical axis. However, misalignment between the coil and the magnet, deflection of the leaf spring frame or the like causes generation of the driving force in an inappropriate direction, that is, a direction to rotate the circuit board. Application of the driving force in such a direction to the rectangular leaf spring frame results in deformation of the whole leaf spring frame.